The present invention relates generally to semiconductor devices and methods for their fabrication. Many steps during the process of semiconductor manufacture involve the deposition or growth of thin dielectric films. The present invention relates to a device and method for determining the conformality of such films by capacitance measurement.
Deposited dielectric films are used as insulators for semiconductor devices including sidewall (vertical surface) isolation. A prime exemplar of such film usage is the gate stack sidewall spacer film used to define the distance between the FET gate stack and self-aligned, lightly-doped source/drain areas, LDD, or extension, and contact silicide regions. The actual sidewall thickness defines the functional geometry of the device, and thus determines device performance and yield. A crucial feature of such films is their conformality, the relation of the thickness of the film deposited on a vertical (sidewall) surface to the thickness on a horizontal surface. A film 100% conformal has a sidewall thickness equal to the deposition depth on a horizontal surface. Currently, there is no practical, direct measure of sidewall thickness, this parameter is determined indirectly from measurements of the thickness of deposited dielectric films along horizontal surfaces relying on the relation of the horizontal thickness (Th), the conformality (C), and the vertical thickness (Tv): Thxc3x97xe2x88x92C=Tv.
Today, ellipsometer measurements of horizontal surfaces are used to control sidewall deposition processes. Moreover, in current monitoring schemes, the conformality is assumed to remain constant. Typically, monitoring is conducted performing a test run of the process using a bare monitor deposited with a planar layer of the film in question, taking ellipsometric readings of the film, and iteratively adjusting process time until desired deposition depth is achieved. Having achieved the predetermined film thickness, production processing ensues.
The control scheme gives assurance that the horizontal coatings are of the appropriate magnitude, but this scheme gives no assurance that conformality has remained unchanged. This is of particular concern in view of the fact that process parameters such as temperature, pressure, and gas flow are known to affect deposition conformality. Where a tool event occurs that alters the process deposition rate, the deposition time is altered to reestablish the predetermined target thickness. However, the present methods do not control for alterations in conformality, and thus the crucial parameter, the sidewall thickness may vary beyond acceptable tolerances.
To improve process productivity, a measure of conformality is essential. There are presently available two methods. The first method, measurement of cross sections by TEM, is barely capable of the requisite 10 xc3x85 resolution, is expensive, has a very long mean time to detect, and its reported measurements are dependent on the TEM operator. The second method measures the electrical Leff or overlap capacitance, Covl, of a device. This method is quite costly and has an unacceptably long mean time to detect, often requiring an additional three to four weeks of wafer processing before measurements can be made.
To control sidewall deposition processes, an inexpensive, short mean time to detect monitor of conformality is required.
Broadly, the invention provides capacitors for which an insulating film whose properties are to be determined serves as the insulation layer. Furthermore the invention provides at least two such capacitors, greatly differing in aspect ratio, paired in such manner that their respective capacitance is dominated by either a horizontal component or a vertical component. Appropriately analyzed, this difference in capacitance allows a determination of the conformality of the insulating film. Moreover, the invention provides that the high and low aspect ratio capacitors may be built on the same wafer or on separate wafers. The invention confers the advantage of a low cost measure of conformality with a very low turn around time.
The invention provides for the monitoring of changes in film conformality during the manufacturing process and permits regulation of the manufacturing process wherein it departs from optimum.
The invention provides means of determining film conformality capable of resolutions below 10 xc3x85 (angstroms).
In addition, the invention provides means of determining film conformality sensitive to the actual conditions that obtain during a production manufacturing run.
Process parameters such as temperature, pressure, gas flow, and tool events are known to affect film conformality. In additional to its other advantages, the invention provides quick and inexpensive means for determining which process parameter(s) are most critical for control of film conformality.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from the following detailed description, wherein it is shown and described preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the description is to be regarded as illustrative in nature and not as restrictive.